Continuous annealing furnace and continuous annealing method for steel strips

Abstract

The invention provides a vertical annealing furnace including a heating zone and a soaking zone without any partition wall therebetween. The furnace has furnace-to-refiner gas suction openings disposed in a lower portion of a joint between the soaking zone and a cooling zone and in the heating zone and / or the soaking zone except a region extending 6 m in a vertical direction and 3 m in a furnace length direction both from a steel strip inlet at a lower portion of the heating zone. The furnace has refiner-to-furnace gas ejection openings disposed in a region in the joint between the soaking zone and the cooling zone, the region being located above the pass line in the joint, and in a region in the heating zone located above 2 m below the center of upper hearth rolls in the vertical direction.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This is the U.S. National Phase application of PCT / JP2013 / 000192, filed Jan. 17, 2013, which claims priority to Japanese Patent Application No. 2012-006994, filed Jan. 17, 2012, the disclosures of each of these applications being incorporated herein by reference in their entireties for all purposes.FIELD OF THE INVENTION[0002]The present invention relates to continuous annealing furnaces and continuous annealing methods for steel strips.BACKGROUND OF THE INVENTION[0003]At start-up of a continuous annealing furnace for the annealing of a steel strip which was once open to the air or in the case when the furnace allows the entry of air into the atmosphere therein, in order to decrease the concentrations of water and oxygen in the furnace, a conventional method that is widely performed is to raise the furnace temperature in order to vaporize the water in the furnace and, almost at the same time, to supply a non-oxidizing gas, for example, an...

AI Technical Summary

Benefits of technology

The present invention is about a continuous annealing furnace for steel strips that can quickly and safely lower the dew point of the atmosphere in the furnace to a level suitable for steady operation, and create a low-dew point atmosphere with little problems of pick-up defects and damage to the furnace walls. This allows for the annealing of steel strips containing easily oxidizable elements such as silicon. Additionally, the invention allows for the stable production of steel strips and prevents a decrease in productivity by rapidly reducing the water and oxygen concentration in the furnace atmosphere to a level where the dew point is lowered to -30°C or below. This prevents the formation of oxides in the steel and makes it easy to manufacture steels that don't favor operation in a high-dew point atmosphere.

Problems solved by technology

However, such conventional methods require a long time to decrease the concentrations of water and oxygen in the furnace atmosphere to prescribed levels suited for steady operation.

Thus, the discontinuation of operation during such a time drastically lowers productivity.

However, high-strength cold rolled steel strips containing easily oxidizable elements such as silicon and manganese have a problem in that these easily oxidizable elements are concentrated at the surface of the steel strips during annealing to form oxides such as of silicon and manganese, deteriorating appearance or chemical conversion property such as phosphatability.

In the case of hot dip galvanized steel strips, the presence of easily oxidizable elements such as silicon and manganese in the steel strips causes a problem that these easily oxidizable elements are concentrated at the surface of the steel strips during annealing to form oxides such as of silicon and manganese, and such oxides impair coating properties to cause the occurrence of bare-spot defects or to decrease the alloying speed during an alloying treatment after the coating process.

In particular, silicon is highly detrimental to coating properties and alloying treatments because a SiO2 film formed on the surface of a steel strip markedly lowers the wettability of the steel strip with respect to a hot dip coating metal and also because a SiO2 film serves as a barrier during an alloying treatment to inhibit the interdiffusion between the base iron and the coating metal.

However, the technique is defective in that it does not allow for efficient production of some types of steel that do not favor being processed in a high-dew point atmosphere (for example, Ti-containing IF steel) because an annealing atmosphere once brought to a high dew point requires a very long time to become one having a low dew point.

In this technique, further, the furnace atmosphere is oxidative and, unless controlled appropriately, causes a problem of pick-up defects due to the attachment of oxides to rolls in the furnace as well as a problem of damage to the furnace walls.

However, because such elements as silicon and manganese are highly prone to oxidation, it has been considered that there will be great difficulties in stably maintaining the atmosphere with a low dew point of −40° C. or below at which excellent suppression is possible of the oxidation of elements such as silicon and manganese, in a large continuous annealing furnace such as one disposed in a CGL (continuous hot dip galvanization line)-CAL (continuous annealing line) system.

These techniques reside in relatively small, single-pass vertical furnaces and are not designed to be applied to multi-pass vertical furnaces such as CGL and CAL systems.

Thus, it is highly probable that these techniques will fail to decrease the dew point efficiently in a multi-pass vertical furnace.

Thus, difficulties are frequently encountered in decreasing the dew point in the entirety of the furnace.

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